Single diode single sideband modulator

ABSTRACT

A transmission line feeds a PIN diode with an information signal. A modulating signal biases the diode so as to switch the diode between forward and reverse bias at an r.f. frequency. The PIN diode exhibits variable capacitance during switching which results in the generation, on the transmission line, of a single sideband modulated signal.

1.- 1: United States Patent 1191 1111 3,739,311 .1 Garver 1 1 June 12, 1973 541 SINGLE DOIDE SINGLE SIDEBAND 2,899,646 3/1959 Read 317/234 VA MODULATOR 3,487,338 l2/l969 Matzelle et al 332/16 T X [75] Inventor: Robert V. Garver, Boyds, Md. 3,319,188 5/1967 Brutsch.... 332/30 V 3,479,615 11/1969 Garver 332/16 Asslgneer The Umted States of America 3,560,883 2 1971 Rogers 332/16 T represented the Secretary of the Army, Washington, DC.

Filed: June 30, 1971 Appl. No.1 158,477

US. Cl 332/16 R, 307/271, 307/317,

Int. Cl H03c 3/22 Field of Search 332/16, 16 T, 45,

332/302,52; 307/320,3l7,27l;3l7/234 VA, 235 AD; 333/1.l; 325/137, 138

References Cited UNITED STATES PATENTS 4/1971 Garver 332/45 l/1969 La Rosa ..333/1.1UX

hie/4 Primary ExaminerAlfred L. Brody Attorney-Harry M. Saragovitz, Edward J. Kelly, Herbert Berl et a1.

[5 7] ABSTRACT 8 Claims, 8 Drawing Figures SINGLE DOIDE SINGLE SIDEBAND MODULATOR The invention described herein may be manufactured, used and licensed by and for the United States Government for governmental purposes without the payment to us of any royalty theron.

FIELD OF THE INVENTION The present invention relates to a single sideband modulators and more particularly to such a modulator employing a PIN diode as the modulating element.

THE PRIOR ART Considering the state of the art, single sideband (SSB) modulators use at least two diodes and some interconnecting r.f. transmission lines to obtain the correct phase condition at each diode. A single sideband modulator of this type is capable of operating at low doppler frequencies and up to a few MHz. However, above 30 MHz the described conventional modulator is inappropriate. A more complete description of this prior art circuit appears hereinafter.

SUMMARY OF THE INVENTION The present invention uses injection-recombination mechanisms inside the I region of a PIN diode to provide the necessary phase conditions for single sideband modulation. It has the advantage over conventional modulators using PIN diodes in that the present circuit operates at higher frequencies. Further, fewer components are required because only a single diode is used with fewer r.f. circuits cooperating therewith. In accordance with the present invention, by modulating a PIN diode with a sine wave at several MHz, the single diode will provide single sideband modulation to r.f. power incident to it.

The above-mentioned objects and advantages of the present invention will be more clearly understood when considered in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a prior art SSB modulator employing dual PIN diodes.

FIG. 2 is a Smith chart which graphically indicates the operation of the circuitry of FIG. 1.

FIG. 3 is a pictoral representation of the carrier density in a PIN diode when operating in the circuit shown in FIG. 1.

FIG. 4 is a pictoral representation of the carrier density in a PIN diode when operating in the circuitry of the present invention.

FIG. 5 is a Smith chart which graphically indicates a mode of operation of a PIN diode in a SSB modulator.

FIG. 6 is a Smith chart which graphically indicates another mode of operation of a PIN diode in a SSB modulator.

FIG. 7 is a plot showing roll off characteristics of a SSB modulator.

FIG. 8 is a schematic of circuitry forming the present invention.

DESCRIPTION OF THE INVENTION Referring to the drawings and more particularly FIG. I, a prior art single sideband modulator is generally denoted by reference numeral 10. Conductors l2 and 14 comprise a transmission line which feeds two PIN diodes 16 and 18 connected in parallel across the transmission line. The anodes of these diodes are directly connected to conductor 12, while the cathodes of these diodes are directly connected to the other conductor 14. Typically, the diodes are separated by a distance of one-eighth wave length. By way of example, the diodes in existing state of the art circuits are driven in phase quadrature. Thus, if the input transmission line has a characteristic impedance of 50 ohms, the line between them must have a characteristic impedance of 100 ohms.

With reference to the Smith chart shown in FIG. 2, when the diodes are biased to 100 ohms, the corresponding position on the Smith chart is point 20, in the center of the chart. It is to be noted that a characteristic of the PIN diode is that its resistance changes with bias. However, the reactance of the PIN diode does not change with bias. As the bias changes, the operating point on the Smith chart changes on the real axis 22.

When moving along the transmission line by a distance of one-eighth wave length, a line of operation is changed from the real axis 22 to a line 24 which has been rotated from the real axis 22. If a second diode is placed in the same plane, a distance of oneeighth wave length closer to the generator, the circuit is returned to an operating line that is horizontal and centered at 26.

The two diodes connected across the transmission line as indicated in FIG. I gives us a new vertical line 28 on the Smith chart, after another 90 rotation of the operating line. Thus, by moving the two diodes apart from one another on the transmission line, a square shaped region is produced with its center at point 20. Accordingly, by driving the diodes in phase quadrature, a circle 30 can be described. By adjusting the input impedance, the circle can be positioned with its center at point 20. This describes, on the Smith chart, single sideband modulation attainable with two PIN diodes.

However, the primary problem involved with utilizing two PIN diodes is the operating frequency limitations. This is due to the fact that injection of carriers in the PIN material requires a determinable period of time. Although this period is relatively short, when two diodes must be considered, there is a cumulative effect of the time requirement for carrier injection. Thus, at high frequencies, this becomes a design limitation.

Referring to FIG. 3, a PIN diode generally indicated by reference numeral 32 is schematically illustrated for the purpose of indicating the junctions of this solid state device. When a PIN diode is switched from reverse bias to forward bias, there is a low drift field applied to the I region 34 and carriers drift into the I region filling it up as schematically indicated in FIG. 3. The carrier density increases as a function of time when the bias condition is switched from reverse to forward. This density increase, as a function of time, is indicated by the progression of levels in the I region (t -t As this occurs, the I region behaves as a resistor whose resistance decreases until a steady state forward bias condition is maintained.

When a large reverse bias is applied to the forward biased PIN diode, a very high field is created immediately at each back biased junction. The carriers drift out of the I region 34 as a wall progressing horizontally across the I region 36 as shown in FIG. 4. Immediately after switching to a reverse bias condition, the expanding depletion region can be characterized as a capacitance going from infinity to some minimum value.

To consider this reverse bias condition in greater desideband modulator, the modulation power must be tail, holes and electrons are present in the I region. carefully considered. When a reverse bias is imposed on the diode, there are Another design consideration is that the upper and electrons in the N region 38 as well as the I region 36. lower sidebands 54 and 56 can be shifted downwardly However, at the I-P junction 40, the electron carriers 5 in frequency as the length of the I region is increased. are swept'to the I region 36. In a similar manner, hole In order to appreciate the physical apparatus of the carriers exist throughout the diode. When reverse bias present invention necessary to achieve total single sideis applied to the diode, the holes begin becoming deband modulation with a single PIN diode, attention is pleted at the I-N junction 42. These carriers will begin directed to FIG. 8. This figure schematically illustrates to move back to the P region. a circulator 55 having a normal input arm 57 through Considering the varying impedance characteristics of which information is transmitted. Arm 58 of the circuthe PIN diode, reference is made to the Smith chart of lator is electrically connected to the PIN diode 60 FIG. 5. The r.f. impedance of the diode during switchthrough a transmission line 61 having a blocking capacing is shown on this chart. itor 62 connected therealong. The anode 64 of diode When the diode is forward biased there is an imped- 60 is connected through an inductor 66 to a modulating ance of 0 ohms. When the diode is reverse biased, the signal. The cathode 68 of the PIN diode 60 is conimpedance is infinite. As the switching goes from renected to ground 70. The modulating signal switches verse bias to forward bias, the horizontal line 44, in the biasing across the diode 60. As a result, in accor- FIG. 5, is followed. When the switching continues from dance with the invention, the output arm 72 of the cirforward bias to reverse bias, the diode acts as a capaciculator carries a normal single sideband modulated sigtance and the impedance changes from an infinite canal. pacitance to 0 capacitance and path 46 on the Smith It should be understood that the invention is not limchart of FIG. 5 is followed. If the switching is to be ited to the exact details of construction shown and dedone at a rpaid rate, the semicircular area bounded by scribed herein for obvious modifications will occur to lines 44 and 46 1S generated and these lines do not cross persons skilled in the art.

one another on the Smith chart. If crossing lines were I claim:

existent on the Smith chart representation, sidebands l. A single sideband modulator comprising a single of different amplitudes would be present. PIN diode means having at least two junctions, means Considering FIG. 6, when a sine wave is applied to a for driving the single diode means through a blocking PIN diode, instead of a square wave, the switching imcapacitor with a carrier signal, means for simultapedance figure takes on the shape shown in FIG. 6. The neously biasing the single diode means through an indotted circle 48 results from a degeneration of the ductor with a modulation signal, said modulation signal semicircular pattern 46 shown in FIG. 5. This degener having a frequency of at least 30 megahertz for varying ation occurs as the frequency changes from a relatively the resistance of the single PIN diode means during' low frequency square wave to a high frequency sine switching of the diode means bias to build up hole carwave. riers, wherein said second-named means and said third- This graphical representation on the Smith chart innamed means combine said carrier signal and said dicates two vectors positioned as radii of the dotted cirmodulation signal to provide, within said single diode cie 48. The vector 50 represents the voltage of re- 40 means, a lower sideband signal and an upper sideband flected carrier power while the vector 52 represents the signal and a relative conversion loss of 20-40 decibels voltage of the reflected single sideband. therein between, means connected to the single diode At low enough frequencies, the diode impedance means for extracting a single sideband modulation sigwould stay on the real axis, and both sidebands would nal, and wherein said diode means consists entirely of be present in equal magnitude. Referring to FIG. 7, as a single PIN diode. frequency is increased, the lower sideband 54 would 2. The circuitry set forth in claim 1 wherein the diode begin to roll off faster than the upper sideband 56. In is characterized as a PIN diode. the case of rapid switching, as discussed in connection 3. The circuitry defined in claim 2 wherein the moduwith FIG. 5, a semicircular pattern is exhibited on the lation signal switches the biasing of the diode in a man- Smith chart, and also as previously mentioned, the sidener causing the diode to exhibit a capacitive impedance bands are of unequal magnitude. The upper sideband characteristic imposing rapid roll-off of an undesired 56 (FIG. 7) would be greater than the lower sideband sideband so as to generate a single sideband signal. 54 for the rapid switching condition discussed. Thus, 4. The subject matter set forth in claim 1 wherein the the conversion loss to the lower sideband 54 begins to means for driving the diode with an information signal decrease as the frequency increases. In the case of ex- 55 is a transmission line connected to an input-output arm tremely high switching frequencies, the conversion loss of a circulator. of upper sideband 56 would tend to decrease due to ti- 5. The circuitry defined in claim 1 wherein the means nite life time, and rectification related to PIN diodes. for biasing the diode with a modulation signal com- However, there is a significant range where the lower prises a signal lead connected to a terminal of the disideband 54 is down while the upper sideband 56 is not. 60 ode, a second terminal of the diode being connected to In fact, there can be a relative conversion loss of 2040 ground.

dB between the two under these circumstances. This 6. The subject matter set forth in claim 4 wherein the condition defines a single sideband modulator. circulator is also provided with a normal input arm for It is of interest, that as modulation power drive is inallowing entry of the informatin sipatto cine diode being connected to ground, the circulator being also provided with a normal input arm for allowing entry of the information signal to the circulator, the means connected to the diode for extracting a single sideband modulation signal from the diode being characterized as an output arm of the circulator. 

1. A single sideband modulator comprising a single PIN diode means having at least two junctions, means for driving the single diode means through a blocking capacitor with a carrier signal, means for simultaneously biasing the single diode means through an inductor with a modulation signal, said modulation signal having a frequency of at least 30 megahertz for varying the resistance of the single PIN diode means during switching of the diode means bias to build up hole carriers, wherein said secondnamed means and said third-named means combine said carrier signal and said modulation signal to provide, within said single diode means, a lower sideband signal and an upper sideband signal and a relative conversion loss of 20-40 decibels therein between, means connected to the single diode means for extracting a single sideband modulation signal, and wherein said diode means consists entirely of a single PIN diode.
 2. The circuitry set forth in claim 1 wherein the diode is characterized as a PIN diode.
 3. The circuitry defined in claim 2 wherein the modulation signal switches the biasing of the diode in a manner causing the diode to exhibit a capacitive impedance characteristic imposing rapid roll-off of an undesired sideband so as to generate a single sideband signal.
 4. The subject matter set forth in claim 1 wherein the means for driving the diode with an information signal is a transmission line connected to an input-output arm of a circulator.
 5. The circuitry defined in claim 1 wherein the means for biasing the diode with a modulation signal comprises a signal lead connected to a terminal of the diode, a second terminal of the diode being connected to ground.
 6. The subject matter set forth in claim 4 wherein the circulator is also provided with a normal input arm for allowing entry of the information signal to the circulator.
 7. The structure of claim 6 wherein the means connected to the diode for extracting a single sideband modulation signal from the diode is characterized as an output arm of the circulator.
 8. The circuitry defined in claim 3 wherein the means for driving the diode with an information signal comprises a transmission line connected to an input-output arm of a circulator, the means for biasing the diode with a modulation signal being a signal lead connected to a terminal of the diode, a second terminal of the diode being connected to ground, the circulator being also provided with a normal input arm for allowing entry of the information signal to the circulator, the means connected to the diode for extracting a single sideband modulation signal from the diode being characterized as an output arm of the circulator. 